ACTION OF OBLIQUE MUSCLES. 59 



away the conjunctival lining of the lower eyelid near the inner part of the 

 orbit, and removing some fat, the muscle will appear beneath the eyeball, 

 bending from the inner to the outer side ; it may be followed outwards to 

 its insertion into the ball. 



The INFERIOR OBLIQUE MUSCLE (fig. 12, n) is situate near the anterior 

 margin of the orbit, and differs from the other muscles in being directed 

 across, instead of parallel to the axis of the orbit. It arises from the 

 superior maxillary bone betwixt the margin of the orbit and the groove 

 for the lachrymal sac. From this spot the muscle passes outwards between 

 the inferior rectus and the bone, and between the eyeball and the external 

 rectus, to be inserted into the sclerotic coat between the outer and upper 

 recti. 



The borders of the muscle look forwards and backwards, and the pos- 

 terior receives the branch of the third nerve. The tendon of insertion is 

 near that of the superior oblique muscle, but rather closer to the optic 

 nerve. 



Action. The oblique muscles rotate the eyeball around an antero-poste- 

 rior axis, and are supposed to be used in maintaining the parallelism of 

 the axes of the two eyes. 



The upper muscle, acting by itself, would rotate the ball in such a way 

 as to cause the inner end of the transverse axis to sink, and the pupil to 

 be directed down and out, as in looking to the shoulder. 



The lower muscle would produce by itself rotation of the ball in the op- 

 posite direction, viz., the inner end of the transverse axis would be raised, 

 and the pupil would be inclined up and out, as in looking to the outer 

 part of the orbit. 



During life the rotary movement is supposed to be chiefly employed in 

 controlling the oblique action of the upper and lower recti. For instance, 

 as the upper rectus alone would turn the pupil upwards and too much in- 

 wards, the iilner movement will be counteracted by the rotation out of the 

 inferior oblique. And as the lower rectus will incline the pupil down and 

 too much in, the motion inwards will be checked by the rotation out of 

 the superior oblique. 



Dissection. To expose the small tensor tarsi muscle, the palpebral liga- 

 ment uniting the eyelids to the margin of the orbit is to be cut through, 

 where this has not been done ; but the lids must be left attached at the 

 inner commissure by means of the tendo palpebrarum. By looking to the 

 posterior aspect of the tendo palpebrarum, after the lids have been placed 

 across the nose, the pale fibres of the tensor tarsi will be recognized. 



The TENSOR TARSI MUSCLE arises from the ridge on the os unguis, and 

 slightly from the bone behind the ridge. Its fibres are pale, and form a 

 very small flat band, behind the ligamentum palpebrarum, which divides 

 like that structure into a slip for each eyelid. In the lid the slip lies by 

 the side of the lachrymal canal, and blends with the fibres of the orbicu- 

 laris along the free margin of the tarsal cartilage. 



Action. By its contraction the muscle draws inwards and backwards 

 the puncta of the lids, so as to favor the reception and passage of the 

 tears. 



Dissection. A small nerve, the temporo-malar or orbital branch of the 

 superior maxillary trunk, lies along the outer angle of the floor of the 

 orbit, and may be brought into view after the removal of the eyeball and 

 its muscles. This nerve is very soft and easily broken, and is covered by 

 pale fleshy fibres (orbitalis muscle). Two branches, temporal and malar, 



